1. Field of the Invention
This invention relates to an observation optical system, and in particular, to an observation optical system for an image display apparatus that can be mounted on a viewer's head or face and can be incorporated in a mobile phone or a personal digital assistant.
2. Description of Related Art
In recent years, for the purpose that each individual person enjoys an image displayed on a large screen, the development of an image display apparatus has taken place. In particular, image display apparatuses of the type mounted on the head or face have been conspicuously developed. The recent widespread use of mobile phones and personal digital assistants has increased the need that data of images and letters of the mobile phones and personal digital assistants should be viewed through large screens. Conventional optical systems used in the image display apparatuses are disclosed, for example, in Japanese Patent Kokai Nos. 2001-264682, 2002-107658, and Hei 11-326824. Each of such optical systems has the structure that a holographic optical element (which is hereinafter abbreviated to HOE) is cemented to a reflecting surface, and the surface of this optical element is decentered and placed. Thus, in each of these prior art publications, a small-sized optical system is proposed.